Phased-array radio frequency receiver

ABSTRACT

A method of RF signal processing comprises receiving an incoming RF signal at each of a plurality of antenna elements that are arranged in a first pattern. The received RF signals from each of the plurality of antenna elements are modulated onto an optical carrier to generate a plurality of modulated signals that each have at least one sideband. The modulated signals are directed along a corresponding plurality of optical channels with outputs arranged in a second pattern corresponding to the first pattern. A composite optical signal is formed using light emanating from the outputs of the plurality of optical channels. Non-spatial information contained in at least one of the received RF signals is extracted from the composite signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Pat. Application No.17/322,156, issued as U.S. Pat. No. 11,515,945, which is a continuationof U.S. Pat. Application No. 16/736,237, filed Jan. 7, 2020, issued asU.S. Pat. No. 11,012,159, which is a continuation of U.S. ApplicationNo. 16/214,998, filed Dec. 10, 2018, issued as U.S. Pat. No. 10,536,220,which is a continuation of U.S. Pat. Application No. 15/790,281, filedon Oct. 23, 2017, issued as U.S. Pat. No. 10,164,712, which is acontinuation of U.S. Pat. Application No. 15/370,035 filed on Dec. 6,2016, issued as U.S. Pat. No. 9,800,346, which is a continuation of U.S.Pat. Application No. 14/791,351 filed on Jul. 3, 2015, issued as U.S.Pat. No. 9,525,489, which claims priority to Provisional PatentApplication No. 62/020,627, the contents of each of which are herebyincorporated by reference in its entirety.

BACKGROUND

The disclosure relates generally to radio frequency (RF) receivers usedto receive and demodulate radio signals, and more specifically to RFreceivers that upconvert signals from RF to optical for signalprocessing.

Conventional RF receivers are limited in dynamic range by spuriousintermixing of signals and/or jamming, either intentional orunintentional.

SUMMARY

Exemplary embodiments provide methods and apparatus for signalprocessing of RF signals received by an array of antenna elements.Methods of RF signal processing comprise receiving an incoming RF signalat each of a plurality of antenna elements that are arranged in a firstpattern. The received RF signals from each of the plurality of antennaelements are modulated onto an optical carrier to generate a pluralityof modulated signals. Each modulated signal has at least one sideband,which may contain information to be recovered or extracted throughfurther processing.

According to aspects of various embodiments, each of the plurality ofmodulated signals is directed along a corresponding one of a pluralityof optical channels, which may for example be optical fibers. Each ofthe optical channels has an output, and the plurality of channel outputsis arranged in a second pattern that corresponds to the pattern of theantenna elements. The plurality of outputs of the optical channels mayemanate light to a free space, or to some other optical channel, inwhich the emanated light forms a composite optical signal. Informationin one or more of the received RF signals, which may include non-spatialinformation, such as, for example, data carried by or encoded onto theRF signal, may be extracted from the composite optical signal.

In some embodiments an image may be formed based on the received RFsignals. A sideband of each of the modulated signals may be isolated,for example by a band-pass filter. Extracting information from thecomposite optical signal may include directing the composite signal ontoa cueing detector and/or onto a signal detector. Extracting informationmay include identifying at least one position of an incoming RF signal,which may include identifying a signal position within an interferencepattern. The angle of arrival of RF signals may be determined in realtime directly from the composite optical signal.

In various embodiments there may be compensation for phase shift in oneor more of the optically modulated RF signals. Compensating for phaseshift may include adjusting an electro-optic modulator.

According to aspects of various embodiments, RF receivers comprise aphased-array antenna that include a plurality of antenna elements and acorresponding plurality of electro-optic modulators. The antennaelements are arranged in a first pattern and configured to receive RFsignals from at least one source. The modulators are configured tomodulate an optical carrier with a received RF signal to generate aplurality of modulated optical signals. A plurality of optical channelscarry the plurality of modulated optical signals and emanate them at acorresponding plurality of channel outputs. The optical channel outputsmay be arranged in a second pattern that corresponds to the firstpattern of the antenna elements.

In various embodiments, RF receivers include a composite signal channel,which may for example be a free space, adjacent the plurality of outputsof the plurality of optical channels. The composite signal channel isconfigured to receive the plurality of modulated optical signals toallow for generating or forming a composite optical signal. A detector,for example a photodiode, may be configured to receive at least aportion of the composite optical signal and to extract non-spatialinformation from a received RF signal.

In some embodiments an RF receiver may include a filter configured toisolate a sideband from at least one modulated optical signal. Thefilter may be located within the composite signal channel.

According to aspects of various embodiments the detector may beconfigured to identify a signal position from the composite opticalsignal. The signal position may correlate with a spatial position of asource of one or more RF signals received by the antenna array. Thedetector may be configured to detect a received RF signal from at leastone source based on the identified signal position. The detector mayinclude a cueing detector configured to use the composite optical signalto identify a signal position that corresponds to a spatial position ofan RF source.

According to other aspects, the RF receiver may further include a phasecompensation detector configured to compensate for phase shifts in themodulated optical signals. The receiver may further include one or moreadditional detectors configured to extract non-spatial information froma received RF signal, and include one or more spatial light modulatorsto direct at least a portion of the composite optical signal onto theadditional detectors. An additional detector may include at least onephotodetector.

According to yet other aspects of various embodiments, methods of RFsignal processing may include receiving an incoming RF signal at each ofa plurality of antenna elements, converting the incoming RF signal ateach of the plurality of antenna elements to a corresponding pluralityof optical signals, directing the optical signals along a correspondingplurality of optical channels, forming a composite optical signal usingoptical signals, detecting a spatial position of a plurality of RFsources from the composite optical signal, and identifying a non-spatialattribute of at least one of the plurality of RF sources based on thecomposite optical signal. The antenna elements may be arranged in afirst pattern and the outputs of the plurality of optical channels maybe arranged in a second pattern that corresponds to the first pattern.Converting the incoming RF signals may include optically modulating atleast one optical signal with the incoming RF signals to generate aplurality of modulated optical signals.

Yet still other aspects include forming an image based on the receivedRF signals. Detecting a spatial position of an RF source may furtheremploy spatial filtering, which may, for example, include nulling anoptical signal that corresponds to an RF signal received by the antennaarray. Some embodiments may include optically steering the compositeoptical signal, for example using optical phase shifting.

In some embodiments, identifying a non-spatial attribute of at least oneof the plurality of RF sources may include receiving a portion of thecomposite optical signal with a photodiode. A portion of the compositeoptical signal may be directed to the photodiode through use ofbeam-splitting of the composite optical signal. The composite opticalsignal may be filtered with a spatial light modulator. Identifying anon-spatial attribute of at least one of the plurality of RF sources maycomprise heterodyning at least a portion of the composite optical signalto generate a heterodyned signal and directing the heterodyned signalonto a photodetector.

Yet still other embodiments provide methods and apparatus for recoveringRF signals from sources and/or extracting information from such RFsignals. An RF signal and/or information contained in an RF signal maybe recovered or extracted by sampling an incoming RF signal from eachsource with a phased-array antenna including antenna elements arrangedin a first pattern. Each sampled RF signal is optically modulated ontoan optical carrier, the optical modulation resulting in a modulatedsignal comprising sidebands flanking the optical carrier. Each of themodulated signals is directed along optical channels, such as opticalfibers. Each optical channel has an output for passing the correspondingmodulated signal to a composite signal channel, such as a free space.The channel outputs are arranged in a second pattern that corresponds tothe first pattern. The optical signals may be filtered to isolate one ofthe sidebands, and an interference pattern formed from the isolatedsidebands originating at the optical channel (e.g., fiber) outputs andpropagating in composite signal channel (e.g., free space). The RFsignal may be recovered from each source and/or information from the RFsignal may be extracted by identifying a signal position within theinterference pattern corresponding to a spatial position of each sourceand detecting the corresponding RF signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure now will be described more fully with referenceto the accompanying drawings, in which various embodiments are shown.The invention may, however, be embodied in many different forms andshould not be construed as limited to the exemplary embodiments. In thedrawings, features may be exaggerated for clarity, and lines withoutarrows may represent bidirectional exchange between components. Likenumbers refer to like elements throughout the drawings, which includethe following:

FIG. 1 is an illustration of an RF receiver in accordance with aspectsof the invention;

FIG. 2 is a block diagram of components for use within the RF receiverof FIG. 1 (and corresponding graphs of signals output by the opticalsource, electro-optic modulator and band-pass filter components) inaccordance with aspects of the invention;

FIG. 3 is a block diagram of additional components for use within the RFreceiver of FIG. 1 in accordance with aspects of the invention;

FIG. 4 is an illustration to aid in defining various quantitiesappearing in the explanation of the operation of the invention;

FIG. 5 is an illustration of an RF scene setup to illustrate calculatingdynamic range enhancement in accordance with aspects of the invention;

FIG. 6 is a graph illustrating dynamic range enhancement as a functionof diffraction efficiency in a spatially-discriminating sparse-arrayreceiver for different numbers of independent image elements inaccordance with aspects of the invention;

FIG. 7 is a block diagram and corresponding results of an experimentaldemonstration of an imaging receiver used to spatially distinguishbetween two active signal emitters in accordance with aspects of theinvention;

FIG. 8 is a block diagram of a single-channel optical reference sourcefor heterodyne detection in accordance with aspects of the invention;and

FIGS. 9A and 9B show flow charts of a method for recovering an RF signalin a radio-frequency phased-array receiver in accordance with aspects ofthe invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments will be described more fully withreference to the accompanying drawings. The inventions as described andclaimed herein may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the disclosure. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting of the inventive conceptdisclosure and claims. As used herein, the singular forms “a,” “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises,” “comprising,” “includes” and/or “including,” whenused herein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art of this disclosure. It will be furtherunderstood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Hereinafter, example embodiments will be explained in detail withreference to the accompanying drawings. The same reference numerals willbe used to refer to the same elements throughout the drawings anddetailed description about the same elements will be omitted in order toavoid redundancy.

Aspects of the disclosure are related to devices and associated methodsfor improving the linear dynamic range and tolerance for jamming in awideband radio-frequency (RF) phased-array receiver. By separatingsignal sources spatially prior to detection/digitization, undesirablenonlinear signal mixing can be reduced or eliminated. Such mixing inconventional receivers can produce spurious intermixing products thatlimit the receiver’s dynamic range, because they cannot be distinguishedfrom genuine signals.

An additional advantage of the embodiments is the ability to determine asignal’s angle of arrival (AoA) in real time. This is unlikeconventional receivers where AoA is determined by a cumbersomecomputation of the cross-correlations between signals from multipleantenna elements after detection and digitization, which result innonlinearities and latency that are detrimental to receiver performance.

Aspects of the embodiments provide a signal detection mechanism whereinRF signals are upconverted by fiber-coupled optical phase modulatorsdriven by the antenna elements of a phased array. The conversion resultsin sidebands on an optical carrier wave supplied by a laser. Theseoptical sidebands are substantially proportional in power to the RFpower incident into each antenna element, and also preserve the phasecarried by the incident RF signal. This essential property of RFupconversion allows the optical sidebands to be used to reconstruct animage of the RF energy in the scene. Dynamic range is improved andresistance to jamming is increased by processing in the optical domain,because energy from separate sources is separated spatially before beingdetected electrically, e.g., by a photodiode or a pixel in an opticalcamera.

A receiver 100 in accordance with aspects of the invention is depictedin FIG. 1 . The illustrated receiver 100 is a sparse-array receiver. Thereceiver 100 includes a processor 200 coupled to the various componentswithin the receiver to implement the functionality described herein.Variations of suitable processors for use in the receiver 100 will beunderstood by one of skill in the art from the description herein.

A phased-array antenna 110, e.g., a sparse array of M antenna elements120 arranged in a predetermined pattern as shown in the example of FIG.1 , receives RF signals from an external source. While the antennaelements 120 shown in FIG. 1 are horn antennae, those of skill in theart understand that a variety of antenna means may be used. RF signalssampled at the antenna elements 120 are used to modulate a laser beamsplit M ways. An electro-optic (EO) modulator 130 is coupled to each ofthe antenna elements 120 and receives a branch of the split laser beamthat it uses to convert the RF energy received at each antenna element120 to the optical domain. It does so by modulating the optical(carrier) beam produced by the laser. The time-variant modulationmanifests itself in the frequency domain as a set of sidebands flankingthe original carrier frequency (or wavelength), at which the sourcelaser operates, as illustrated in FIG. 2 , which is discussed in moredetail below. As a result, the energy radiated in the RF domain appearsin the optical domain as sidebands of the carrier frequency. Thisup-conversion of the RF signal into optical domain is coherent in thesense that all the phase and amplitude information present in RF ispreserved in the optical sidebands. This property of coherencepreservation in optical up-conversion allows the recovery of theRF-signal angle of arrival using optical means.

Returning to FIG. 1 , the modulated optical beams containing the lasercarrier wavelength and the sidebands with imprinted RF signal areconveyed by optical fibers 140 to a lenslet array 150 coupled to theoutputs 141 of the fibers 140 that are arranged in a second pattern thatmimics or corresponds to the first pattern of the array of the RFantennas, at a reduced scale. FIG. 4 illustrates the output ends of theoptical fibers 140 arranged in a pattern that corresponds to the patternof the antenna elements 120 of FIG. 1 . As illustrated in FIG. 4 , fromthe outputs 141 of the optical fibers 140 at the lenslet array 150 on,the beams propagate in free space, no longer guided by the opticalfibers. While the embodiment of FIG. 1 shows conventional optical fibers140 between the electro-optic modulators 130 and the processor 200,those of skill in the art will appreciate that other optical waveguidesor channels may also or instead be used (as illustrated in FIG. 3 ).Similarly, while FIGS. 3 and 4 illustrate the use of a free space in theprocessor 200 as a channel for forming a composite optical signal fromlight emanating from the outputs of the optical fibers 140, thoseskilled in the art will appreciate that other optical channels can beused for forming a composite optical signal.

Again referring back to FIG. 1 , the individual beams propagate in freespace from the outputs 141 of fibers 140 at the lenslet array 150, whichallows the individual beams to interfere with one-another where theyoverlap to form a combined or composite beam 160. Part of the combinedbeam 160 is split off with a beam-splitter 165, mixed with a referencebeam 170, and sent to an array of detectors 175 (phase-compensationdetectors in FIG. 1 ) in order to detect, and allow for the compensationof, optical phase variation originating in the individual fibers 140 dueto environmental conditions such as vibrations and acoustics. Thisensures that the resulting image corresponds to spatial distribution ofRF sources in the scene as opposed to vibrating fibers. A band-passoptical filter 180, see FIG. 1 , strips off the carrier wavelength andallows only one of the sidebands through, FIG. 2 . The overlapping beamsthat now carry only a single sideband are projected onto a cueingdetector 190, e.g., a charge coupled device (CCD) array, where theyinterfere to form a representation of the RF angle of arrival in theoptical domain. In other words, the optical image formed by theoverlapping beams on the cueing detector 190 may substantially be areplica of the RF scene as seen by the sparse antenna array 110.

FIG. 2 illustrates the use of an optical filter 180 to recover orisolate an optical sideband that corresponds to a received RF signal,which may for example be a millimeter wave (MMW) signal having afrequency ω_(m). As shown in the graphs of FIG. 2 , the received RFsignal(s) from antenna element(s) 120 modulate with an optical carriersignal (source) 125 operating at a frequency ω₀ (illustratively at awavelength between 1557 and 1558 nm). The output 135 of modulator 130includes an optical analog of the MMW signal in sidebands of the opticalcarrier as shown in the middle graph. An optical band-pass filter 180tuned to ω₀ + ω_(m) or ω₀ - ω_(m) strips off (isolates) the opticalrepresentation of the received MMW signal(s) from the carrier.

FIG. 3 depicts the configuration of a receiver 100 with an emphasis onthe optical layer. The single laser source 125 is split M ways by asplitter 127 and the beams 128 are routed through modulators 130 coupledto antennas 120 capturing the RF radiation. The (optical) outputs 135 ofthe modulators 130 are filtered to allow only a single sidebandcorresponding to the captured RF radiation to pass, for example using afilter 180 as described with FIG. 2 . The free-space interference of theoptical beams 185 output from filter 180 among the M different channelsyields a pattern measured with detectors, as discussed in more detailbelow. Mixing the interference pattern produced by the outputs 185 withreference beam(s) 187 allows for the extraction of information carriedin the optical beam(s) modulated with incoming RF signal(s).

Note that FIGS. 1 and 3 depicts the filter 180 positioned in thefree-space portion of the receiver 100 downstream of the lenslet array150. In alternative embodiments, the filter can be placed anywherebetween the modulators 130 and the cueing detector 190 to enablereconstruction of the RF-source position in the optical domain.Furthermore, in some embodiments, especially for frequencies lower than~5 GHz, a Mach-Zehnder modulator (MZM) may be used for filter 180 tofilter out the sideband energy from the optical carrier energy. Suchmodulators can, under appropriate bias conditions, interferometricallysuppress the carrier while passing the (odd-ordered) sidebands, therebysuppressing the carrier in a frequency-independent manner.

The cueing detector 190 of FIG. 1 may be an array of photo-detectorssuch as those of a charged coupled device (CCD) or contact image sensoror CMOS image sensor, which in some embodiments may not be able todecode information present in the RF signals received by the antennaarray 110 with the same performance as high-speed photodiodes. In someembodiments, to extract or recover information encoded in the RF signalsinput by the antenna elements 120, the composite optical beam outputfrom filter 180 is further split with additional beam-splitters 165, andcombined with reference laser beams 192 for heterodyne detection at fastphotodiodes 194 as illustrated in FIG. 1 . A few examples of non-spatialinformation encoded into an RF signal that may be detected byphotodiodes 194 include amplitude, phase, and/or frequency modulation ofan RF carrier with information-bearing signal. The information-bearingmodulating signal may be analog or digital in nature. In the lattercase, the information may be contained in frequency-divisionmultiplexed, time-division multiplexed, or code division multiple accesssignals (FDM, TDM or CDMA respectively; using telecommunication examplesfor more specificity for each, e.g., OFDM, GSM, or WCDMA signals). Forexample, each photodiode 194 may receive an OFDM signal comprisingmultiple carrier signals that are orthogonal to each other. Each of themultiple carrier signals may be appropriately demodulated (e.g., tobaseband) to extract data (e.g., a digital data comprising binary bitsof 0’s and 1’s). Each OFDM signal received by each photodiode 194 maycomprise multiple channels of data, each associated with a differenttransmission (e.g., each associated with a different audio signal ordifferent video signal). As is known, a channel of digital data need notbe carried by a single carrier but may be spread across multiple ones ofthese carriers (e.g., via frequency hopping or interleaving). The RFcarriers of the OFDM signals simultaneously transmitted by the RFsources and received by each photodiode 194 may have same frequencies;interference amongst the simultaneously received OFDM signals may beavoided due to the spatial separation of the RF sources. Each OFDMsignal received and demodulated by each photodiode 194 may correspond toan OFDM RF signal transmitted by one or more of the RF sources andreceived by antennas 120 (e.g., in the millimeter wavelength RF range,or in a range of 3 to 300 GHz, or between 0.5 to 300 GHz, such as0.5-110 GHz, or in the HF band of 3 to 30 MHz, or in VHF band of 30 to300 MHz, or in UHF band of 300 MHz to 1 GHz). Thus, for example,antennas 120 may receive multiple OFDM signals each having multiplechannels to carry multiple transmissions of digital data on multiplesignal carriers, such as digital audio (e.g., MP3, MPEG), digital imagesdigital video (e.g., MP4), data in TCP/IP format, etc. Opticalconversion and processing (as described herein) may provide each ofthese OFDM signals emanating from different RF transmitters to adifferent corresponding photodiode as a converted optical signal.Although the above example describes transmission and receiving of oneor more OFDM signals, other RF encoding/decoding schemes (as notedherein) may be utilized and processed in the optical domain in a similarmanner. Spatial light modulator (SLM) phase shifters 196 ensure that thesources of RF radiation, detected as bright spots in the cueing detector190, are imaged on the fast photodiodes 194 individually. In FIG. 1 twosuch fast photo-diodes 194 with the corresponding SLMs 196 are shown forillustration; they allow receiving signals from two distinct RF sourcessimultaneously. Increasing the number of photo-diodes 194 with thecorresponding SLMs 196 and beam splitters 165 increases the number ofreceived RF signals that can be processed simultaneously to extract orrecover information.

In alternative embodiments, an array of suitably fast photo-detectorscan be used in place of the relatively slow CCD in the cueing detectorillustrated in FIG. 1 . Upon mixing with optical references, suchalternative detector array embodiments provide means for both spatialdiscrimination of the RF sources, and extracting information carried bythe corresponding RF signals. In these alternative embodiments, theadditional beam splitters 165, SLMs 196 and photodiodes 194 shown inFIG. 1 are unnecessary.

Below, further details on the optical reconstruction of the RF scene arepresented. To reconstruct the image of the RF scene in the opticaldomain, the (optical) outputs of the modulators 130 are carried inoptical fibers 140 to a lenslet array 150 that mimics the spatialdistribution of the antennas 120. The output beams are then allowed tointerfere in free space (or other suitable channel for forming acomposite optical signal), and the interference pattern corresponding tothe original RF scene is captured by an array of optical sensors such asa CCD chip embodiment of cueing detector 190. In the absence of spectralfiltering, the image reconstruction process can be expressed as follows:

$C_{n} = \frac{1}{\sqrt{N}}{\sum\limits_{m = 0}^{M - 1}{B_{m}e}}^{i{({\omega t + \theta_{nm} + \varphi_{m}})}} + \text{c}\text{.c}\text{.}$

where, with reference to FIG. 4 , B_(m) is the amplitude of the field atthe output of the m-th fiber, C_(n) is the electric field (of light) atthe n-th pixel of the CCD (in the absence of spectral filtering), ω isthe optical frequency, φ_(m) is the (RF-modulated) phase of the opticalbeam in the m-th fiber, and θ_(nm) is the phase the optical beam picksup as it propagates in free space from m-th fiber to n-th pixel; it isassumed that there are M optical fibers, N sensing elements in the CCDarray, and that the intensity of light coming out of each fiber isevenly distributed among the N sensors of the CCD; c.c. signifies thepresence of complex conjugate of the first term that makes the electricfield a real number. As noted, the beams output by the optical fibers140 are allowed to interfere in an interference providing space (afiberless space). Such an interference space may be transparent and maycomprise a vacuum, air, a gas other than air, a liquid or a solid (e.g.,a lens or a slab waveguide).

For the purpose of the following analysis, the RF (e.g., mmW) scene isdivided into discrete RF emitters enumerated with index k. The phaseimposed on the optical carrier in the m-th channel by k-th RF emitter is

S_(k)cos (Ωt + ϕ_(km)),

where Ω is the frequency of the RF signal, S_(k) is the amplitude of thewave emitted by the k-th emitter, scaled by modulation efficiency andthe distance from the aperture, and ϕ_(km) is the phase picked up by thewave between the k-th emitter and the m-th antenna element of the array.The total phase in the m-th channel is obtained by adding contributionsfrom all RF sources in the scene, i.e.

$\varphi_{m} = {\sum\limits_{k}{S_{k}\cos\left( {\Omega t + \phi_{km}} \right)}}$

If the RF waves originating at different positions are uncorrelated, itcan be shown that Eqs. (1), (2) and (3), in combination with spectralfiltering that allows only one sideband through, yield the followingaverage power detected at the n-th pixel of the CCD array

$P_{n} = {\sum\limits_{m,m'}e^{- i\text{k}_{n} \cdot {({\text{x}_{m} - \text{x}_{m'}})}}}{\sum\limits_{k}{S_{k}^{2}e^{i\text{K}_{k} \cdot {({\text{X}_{m} - \text{X}_{m'}})}}}}.$

Equation (4) has a form of a composition of Fourier and inverse-Fouriertransformations, and therefore, it spatially reconstructs the positionsof the RF sources present in the scene as bright spots on the CCD array.In Eq. (4), K_(k) is the wave-vector of the RF wave associated with k-thsource, X_(m) is the position of the m-th antenna in the array, x_(m) isthe position of the m-th fiber in the array, and k_(n) is thewave-vector of the optical wave-form produced by the fiber array that iscollected by the n-th pixel in the CCD array.

The information of the positions of the sources of RF radiation obtainedthis way from the cueing detector 190 is then used in the SLM phaseshifters 196 to project the regions of interest onto fastphoto-detectors 194, which, with the help of a heterodyne opticalreference 192, convert the modulated light back into RF for furtherprocessing.

FIG. 9A depicts a flow chart 10 of steps for spatial discrimination ofRF sources and the corresponding signal detection in a radio-frequencyphased-array receiver according to aspects of the inventions. The stepsof flow chart 10 may be performed using the receiver depicted in FIG. 11as well as a wide variety of other embodiments that would be apparent tothose of skill in the art.

At step 12, the incoming RF signal is received (or sampled, etc.), e.g.,by a phased-array antenna. The incoming RF signal from each of at leastone source may be sampled with a plurality of antenna elements in aphased-array antenna. The phased-array antenna may be arranged in afirst pattern.

At step 14, an optical carrier is modulated with the received RF signal.An optical carrier may be modulated by each of the at least one RFsignal received by each of the plurality of antenna elements with acorresponding electro-optic modulator. The optical modulation of theoptical carrier with the RF signals results in a modulated optical beamcomprising at least one sideband flanking the optical carrier.

At step 16, each of the modulated beams may be directed along an opticalchannel, e.g., an optical fiber. Each optical fiber has an output forpassing its corresponding modulated signal to a composite signalchannel, such as a free space, in which a composite optical signal willform from combined outputs. The outputs of the plurality of opticalfibers may be arranged in a second pattern corresponding to the firstpattern, wherein propagation of the optical beams from the outputs intofree space forms an interference pattern.

At step 18, each of the RF-modulated optical signals is filtered toisolate one of the sidebands.

At step 20, information contained in at least one RF signal is recoveredor extracted. The RF signal information may be recovered by identifyinga signal position within the interference pattern corresponding to aspatial position of the source of the RF signal. Non-spatialinformation, such as information encoded onto the RF signal thatcorresponds to that signal position, may be detected or extracted fromthe corresponding modulated optical signal.

FIG. 9B depicts a flow chart 20 of steps detailing the process for therecovery or extraction of information of the RF signal(s). The steps offlow chart 20 may be performed using the receiver depicted in FIG. 1 ,although those of skill in the art will understand a variety of otherembodiments are suitable for performing the steps.

At step 22, the signal positions are detected by a first detector. Forexample, the interference pattern may be directed onto a cueing detectorto identify the signal positions where each identified signal positioncorresponds to the spatial position of an RF source.

At step 24, the non-spatial information of the RF signals is extractedor recovered from the corresponding modulated optical signals. Theinterference pattern may be directed onto a signal detector with aspatial-light-modulator, using the signal positions identified in step22, to extract or recover the information from the RF signals from atleast one source.

In another embodiment, the RF signal may be recovered by directing theinterference pattern onto a signal detector that identifies the signalpositions, where each identified signal position corresponds to thespatial position a source. The same signal detector additionally detectsthe RF signals from each of the at least one source at the identifiedsignal positions within the interference pattern.

In preferred embodiments incorporating multiple high-speedphotodetectors, each of the fast photodetectors receives power from onlyone element of the scene-from one RF source—while effectivelysuppressing all other sources that may be present. Below, issues relatedto such mapping are quantified, and expressed in terms of theenhancement of effective dynamic range.

Spatial filtering may be employed to improve effective dynamic range.The spatial separation of the RF radiation arriving from differentdirections prior to electronic processing provides means for suppressingunwanted (jamming) sources as long as they are not collocated with theregion of interest. Such suppression is equivalent to effectiveenhancement of the dynamic range: the receiver is capable of detecting aweaker signal in the presence of a stronger source than would otherwisebe possible in a conventional configuration.

Specifying certain functional characteristics of the receiver canquantify this enhancement. First is the number of independent elements,N, of the reproduced image of the RF scene. Essentially, N is equal tothe field of view of the antenna array divided by the resolution.Another way to look at the number of independent elements is by usingconcepts developed by Claude Shannon in the context oftelecommunications. The time-bandwidth product, which is equal to thedimension of the space of all possible messages that can be transmittedin a given channel over a certain bandwidth in a given time, plays acentral role. The analogue of the time-bandwidth product in the case ofimaging with a 2D aperture is the area-spatial-frequency-bandwidthproduct. To calculate spatial-frequency-bandwidth, the frequency (orwavelength) and the field of view are needed. The spatial frequencycaptured by the aperture is obtained by projecting the incident k-vectoron the aperture plane. The higher the incidence angle at a givenfrequency, the higher the spatial frequency. Thus, for a square apertureand square field of view, ±θ in each direction, thespatial-frequency-bandwidth is

$\left\lbrack {2\frac{v\sin(\theta)}{c}} \right\rbrack^{2},$

where v is the frequency of the received RF signal, and c is the speedof light. Assuming a square aperture with side L, and the respectivearea L², the number of independent image elements is

$N = \left\lbrack {2\frac{vL\sin(\theta)}{c}} \right\rbrack^{2}.$

Another concept required for the evaluation of the equivalent dynamicrange enhancement is the diffraction efficiency η of the antenna array.In the language of diffractive optics, the diffraction efficiencymeasures the fraction of the overall received power that ends up in thedesired location or direction. In the context of the embodimentsdisclosed herein, it is useful to consider a distant point sourceilluminating the antenna array. On the imaging side, in the cueingdetector, the point source becomes an image consisting of a spike at onelocation, and some increased background level elsewhere. In other words,in addition to the one desired image element, the point source alsoilluminates all other N-1 image elements to some extent. The ratio ofthe power received in the desired element to the total power received isthe diffraction efficiency η.

The number of elements that can be realized on a given platform islimited to its maximum physical extent. As an example, for a source at 3GHz the number of resolvable elements would be limited to approximatelyN=400 for a 1-meter aperture into a 2 π steradian solid angle field ofview. At 106 GHz, the same aperture affords N=500,000.

To calculate the enhancement of the effective dynamic range, consider acase depicted in FIG. 5 where the signal of interest comes from theregion of interest denoted as o in the presence of a strong sourcemarked as *. As indicated above, the strong source, in addition toproducing a bright spot in the respective image location, will depositenergy in all other locations including the region of interest. Assumingthat the total power coming from the strong source is P*, the powerdelivered to point * is ηP*. Therefore, the total power delivered to allimage elements excluding * is (1-η)P*. Assuming that the power from thestrong source that misses the mark is evenly distributed among theremaining N-1 elements, the region of interest o receives

$\frac{1 - \eta}{N - 1}P*$

as its share. At the same time, fraction η of power P^(o) originating atthe region of interest is deposited at the point corresponding to o.Therefore, the ratio of the desired power to the undesired power at theobserved region of interest is

$\frac{\eta}{1 - \eta}\left( {N - 1} \right)\frac{P^{\text{o}}}{P*}.$

In the absence of spatial filtering, each antenna element of the arrayreceives the total power originating at the entire field of view andpasses all of it for electronic processing. Therefore, the ratio of thedesired power to the undesired power that need to be discriminated is

$\frac{P^{\text{o}}}{P*}.$

By comparing expressions (8) and (9), the enhancement of the desired tothe undesired power ratio that needs to be processed electronically isobtained as

$\frac{\eta}{1 - \eta}\left( {N - 1} \right).$

Formula (10) may be interpreted as the effective enhancement of thedynamic range of the receiver. Such interpretation is justified by theway it was derived as the ratio of ratios of desired to undesired powersthat need to be processed electronically. In other words, all else beingequal, the receiver can tolerate the level of ‘jamming’ power increasedby a factor η(N-1)1(1-η) and deliver the same performance in terms ofdetecting the desired signal as a conventional receiver configuration.

The dependence of the dynamic range enhancement as a function ofdiffraction efficiency for several different values of N is shown inFIG. 6 . Note that the minimum enhancement is always 0 dB. This can beascertained by noting that the minimum diffraction efficiency occurswhen the incoming power is evenly spread among all N elements of the RFsignals captured by the antenna array, i.e. η = ⅟N. Substituting thisvalue to expression (10) yields 1, or, equivalently 0 dB. On the otherhand, at high diffraction efficiency approaching 1, which corresponds tofully populated antenna arrays, the dynamic-range enhancement isdivergent-it tends to infinity. Such behavior may intuitively beunderstood by noting that at high diffraction efficiency, negligibleamounts of power leak to image elements that do not correspond to theposition of the source, i.e., the region of interest o receivesnegligible power from source *. As a result of this almost ideal spatialfiltering, the ability to jam the receiver by using a spatiallyseparated strong source may be effectively eliminated.

An assumption of this analysis is that the power from the unwantedsource is evenly distributed across the other elements of the array. Inactuality, this distribution will be non-uniform, with nulls that can beadjusted to further reduce contributions from unwanted sources. Thus,using smart nulling techniques that are known to those of skill in theart, embodiments can provide significant improvement in dynamic rangebeyond the nominal the results described above.

Tests to demonstrate aspects of embodiments of the inventions have beenperformed, as discussed below.

FIG. 7 depicts a schematic illustration and representative preliminarydata obtained from a test configuration 400 to demonstrate spatiallyseparating signals via optical upconversion and imaging prior toelectronic detection. An array of receivers 410 comprising opticalmodulators (130) driven by antennas (120) was used to obtain an image oftwo ~35-GHz emitters (34.5-GHz left emitter 401 and 35.0-GHz rightemitter 402). The system used an image reconstruction in the opticaldomain by the camera 420, in parallel with optical phase steering byapplying DC bias to optical modulators (130 in FIGS. 1 and 2 ) to directenergy from the emitters onto a high-speed photodetector 440. Thephotodetector 440’s output spectra were measured by an RF spectrumanalyzer 450 and are shown at the top right of FIG. 7 . The imageobtained by the system is shown at the lower right of FIG. 7 , andconsists of two bright spots. After upconversion, but before detectionby the camera, a beamsplitter 460 was used to separate a portion of theoptical energy and direct it to the high-speed photodetector 440.Optical phase shifting in each of the modulators 130 was used to steerthe image of either emitter to the photodetector. Electricalspectrum-analyzer traces of the detector output are shown in the upperright region of FIG. 7 : The top spectrum is for the case when the imagewas steered to direct the energy of the left emitter 401 operating at34.5 GHz onto the photodetector 440 whereas the bottom spectrumcorresponds to steering the image so as to direct the energy of theright emitter 402 operating at 35.0 GHz onto the photodetector 440. Thetwo sources 401, 402 operated at the same power levels for the captureof the two spectra. The difference in the two spectra, in particular thedifference in the detected relative intensities between the twodifferent sources, is the result of spatial discrimination between thetwo sources as afforded by the test embodiment which implemented aspectsof the inventions.

In the case of this test demonstration, the sidebands were detected bybeating with a residual of the optical carrier frequency, henceelectrical signals were obtained directly at the frequencies of theemitters. An additional aspect of some embodiments, as illustrated inFIG. 8 , is the use of a coherent optical local oscillator (LO) forconverting the sideband energy to a more accessible intermediatefrequency (IF). This heterodyne technique allows the detection ofsignals over a widely tunable frequency range, and also eases thebandwidth required of detectors such as photodiode(s) (for example,photodiodes 194 of FIG. 1 ). A single (master) laser (510) may be usednot only to feed all the modulators in the array, but also to generatethe coherent heterodyne optical LOs according to the scheme shown inFIG. 8 and described below. As depicted in FIG. 1 , beam-splitters 165and spatial light modulators (SLMs) 196 may then be used to directspatially filtered signals from their respective positions in the imageplane onto photo-diodes 194 for heterodyne detection. The optical LOgeneration technology is based on modulation-sideband injection lockingof semiconductor lasers, offering enormous bandwidth, superb signalpurity via cancellation of optical phase noise, and a minimal sizeweight and power (SWaP) due to the use of optical fibers and photoniccomponents.

Wide tunability is realized by injection locking using a broad comb ofharmonics, all derived from externally modulating Laser 1 (510) with alow-frequency RF reference 520 that has been subject to nonlineardistortion as shown in FIG. 8 . The output of Laser 1 is also used asthe optical carrier in the description above. Laser 2 (530) is tuned tomatch and lock to the frequency of any one of the injected harmonics.Choosing higher harmonics allows very high offset frequencies to beobtained, and because the locked lasers have identical phase noise, thepurity of the reference is preserved. Continuous fine-tuning isavailable from a tunable reference, e.g., the voltage controlledoscillator (VCO) 520. This approach has been demonstrated to provide forcontinuous tuning over at least 7 octaves (0.5-110 GHz), with a measuredlinewidth of ~1 Hz over that entire range. This approach enablescontinuous tuning up to and exceeding 300 GHz by use of improvedmodulator technology.

Notably, the photo-diode for RF signal recovery, broadband photodiodes194 in FIG. 1 , need only possess sufficient speed for the intermediatefrequency plus the signal bandwidth, not the RF carrier frequency, sothe tuning range of the receiver is not photo-diode limited, and canextend to the full range of the modulator’s operation. This featureallows use of photodiodes with higher optical power handling and outputphotocurrents. Using such photodiodes in this architecture allowssignals to be received with net gain, and with spur-free dynamic range(SFDR) in excess of 120 dB Hz^(2/3). Note that this estimate does notinclude the improved dynamic range afforded by spatial filtering asdescribed above.

The foregoing is illustrative of exemplary embodiments and is not to beconstrued as limiting thereof. Although a few exemplary embodiments havebeen described, those skilled in the art will readily appreciate thatmany modifications are possible without materially departing from thenovel teachings and advantages of the inventive concepts. Accordingly,all such modifications are intended to be included within the scope ofthe present invention as defined in the claims.

What is claimed is:
 1. A method of RF signal processing comprising:receiving an incoming RF signal at each of a plurality of antennaelements that are arranged in a first pattern; modulating the receivedRF signals from each of the plurality of antenna elements onto anoptical carrier to generate a plurality of modulated signals, eachmodulated signal having at least one sideband; directing each of theplurality of modulated signals along a corresponding one of a pluralityof optical channels, each of the plurality of optical channels having anoutput, the outputs of the plurality of optical channels arranged in asecond pattern corresponding to the first pattern; forming a compositeoptical signal using light emanating from the outputs of the pluralityof optical channels; and extracting, from the composite optical signal,non-spatial information contained in at least one of the received RFsignals.
 2. The method of claim 1, further comprising forming an imagebased on the received RF signals.
 3. The method of claim 1, wherein theoutputs of the plurality of optical channels emanate light to a freespace.
 4. The method of claim 1, further comprising isolating a sidebandof each of the modulated signals.
 5. The method claim 1, wherein theextracting step comprises directing the composite signal onto a cueingdetector.
 6. The method of claim 1, wherein the extracting stepcomprises directing at least a portion of the composite signal onto asignal detector and heterodyning it with an optical reference signal. 7.The method of claim 1, wherein the extracting step comprises identifyingat least one position of an incoming RF signal.
 8. The method of claim7, wherein identifying at least one position comprises identifying asignal position within an interference pattern.
 9. The method claim 1,further comprising compensating for a phase shift in at least one RFmodulated optical signal.
 10. The method of claim 9, whereincompensating for a phase shift comprises adjusting an electro-opticmodulator.
 11. An RF receiver comprising: a phased-array antennaincluding a plurality of antenna elements arranged in a first patternconfigured to receive RF signals from at least one source; a pluralityof electro-optic modulators corresponding to the plurality of antennaelements, each modulator configured to modulate an optical carrier witha received RF signal to generate a plurality of modulated opticalsignals; a plurality of optical channels configured to carry theplurality of modulated optical signals, each of the plurality of opticalchannels having an output to emanate the corresponding modulated opticalsignal out of the corresponding optical channel, the outputs of theplurality of optical channels arranged in a second pattern correspondingto the first pattern; a composite signal channel, adjacent to theplurality of outputs of the plurality of optical channels, configured toreceive the plurality of modulated optical signals, whereby a compositeoptical signal is generated; and a detector configured to receive thecomposite optical signal and to extract non-spatial information from areceived RF signal.
 12. The RF receiver of claim 11, further comprisinga filter configured to isolate a sideband from at least one modulatedoptical signal.
 13. The RF receiver of claim 12, wherein the filter islocated within the composite signal channel.
 14. The RF receiver ofclaim 11, wherein the composite signal channel comprises a free spaceadjacent the outputs of the plurality of optical channels.
 15. The RFreceiver of claim 11, wherein the detector is configured to identify asignal position from the composite optical signal.
 16. The RF receiverof claim 15, wherein the signal position correlates with a spatialposition of the at least one source.
 17. The RF receiver of claim 15,wherein the detector is further configured to detect a received RFsignal from at least one source based on the identified signal position.18. The RF receiver of claim 11, wherein the detector comprises a cueingdetector configured to use the composite optical signal to identify asignal position that corresponds to a spatial position of an RF source.19. The RF receiver of claim 11, further comprising: a second detector;and a spatial-light-modulator configured to direct the composite opticalsignal onto the second detector.
 20. A method of RF signal processingcomprising: receiving an incoming RF signal at each of a plurality ofantenna elements that are arranged in a first pattern; converting theincoming RF signal at each of the plurality of antenna elements to acorresponding plurality of optical signals by optically modulating atleast one optical signal with the incoming RF signals to generate aplurality of modulated optical signals; directing each of the pluralityof modulated optical signals along a corresponding one of a plurality ofoptical channels, each of the plurality of optical channels having anoutput, the outputs of the plurality of optical channels arranged in asecond pattern corresponding to the first pattern; forming a compositeoptical signal using optical signals from the outputs; detecting aspatial position of a plurality of RF sources from the composite opticalsignal; and identifying a non-spatial attribute of at least one of theplurality of RF sources based on the composite optical signal.